Abstract
Three groups of rats were given brief, inescapable shocks in a shuttlebox on the average of one every 5, 10, or 20 sec., respectively. Each group was divided into four subgroups, able to reduce the probability of occurrence of the next shock by 75%, 50%, 25%, or not at all, respectively. For all initial shock rates a greater reduction in the probability of occurrence of shock led to a higher percentage of avoidable shocks being avoided. The ratio of non-shock-elicited responses to shocks increased for initial shock rates of one per 5 and 10 sec. when associated with greater reductions in shock probability but the ratio decreased for an initial shock rate of one per 20 sec. for greater reductions in shock probability. The most plausible explanation is in terms of the stimulus control of behavior similar to that invoked for operant behavior under multiple probabilistic scheduling of positive reinforcement.
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